As conventionally known methods for forming a coil, there is a method of forming a plurality of coil patterns, each of which will be a part of a coil, in a circuit board and then connecting these coil patterns electrically so as to form a coil. For instance, as shown in FIG. 9, JP 2001-77538A suggests that coil patterns 402a to 402d shaped like a letter C are formed on principal surfaces of respective layers 401a to 401c of a build-up multilayer board 400, and the coil patterns 402a to 402d are connected via build-up vias 403a to 403c so as to form a circuit board including a patterned coil with a spiral shape as a whole.
Furthermore, as shown in FIG. 10, JP 2000-223316A suggests a circuit board having a coil structure formed as follows: a plurality of coil patterns 501 and 502 are formed to be aligned on upper and lower surfaces 500a and 500b of a board 500, and end portions 501a and 502a of the respective coil patterns 501 and 502, provided backward (upper right side) of this drawing, are connected electrically with each other via a plurality of conductive portions 503a formed to penetrate the board 500. Similarly, end portions 501b and 502b of the respective coil patterns 501 and 502, provided forward (lower left side) of this drawing, are connected electrically with each other via a plurality of conductive portions 503b formed to penetrate the board 500.
According to these circuit boards, there is no need for separately mounting a coil on the circuit board, thus enhancing the working efficiency during the manufacturing of a circuit.
However, for the patterned coil described in JP 2001-77538A, the coil patterns are provided on principal surfaces of the respective layers of the board. Therefore, in order to increase the winding number of the coil, the number of layers of the board has to be increased. Although the winding number of the coil has to be increased for obtaining a larger inductance, the winding number of the above-mentioned patterned coil is restricted to the number of the layers of the board as described above. Therefore, if an inductance larger than a predetermined threshold value is required, it may be difficult to increase the winding number. Also, when the number of the layers of the board is increased for obtaining a larger inductance, displacement in the lamination may occur. In an extreme case, the connection of build-up vias with coil patterns formed between the layers may become unstable, resulting in electrical discontinuity.
On the other hand, the coil structure described in JP 2000-223316A is wound in the direction perpendicular to the thickness direction of the board, and therefore the winding number of the coil can be increased in this winding direction. However, since the board is a so-called double-sided circuit board including a single layer of an electrical insulative base member and conductive patterned layers formed on both principal surfaces of this electrical insulative base member, there is a possibility of lowering the flexibility of circuit design. For instance, since a single-winding coil only can be provided in the thickness direction of the board, if an inductance larger than a predetermined threshold value is required, it may be difficult to increase the winding number. Further, there is a possibility of the failure to provide members having various functions, which will described later, including a shielding function, a current detection function and the like, separately.